Rocket propelled flight vehicles typically include mechanisms to provide control of the flight vehicle's orientation (attitude) about its center of gravity (CG). While in flight, the flight vehicle rotates in three dimensions about three axes running through the center of mass of the vehicle. The three angles of rotations are known as roll, pitch and yaw. Most flight vehicles are symmetric about a central longitudinal axis running from nose to tail through the CG. Although the CG is typically referred to when flying in the atmosphere, referencing the CG herein may also be used to refer to the center of mass of vehicles in space when there is effectively no gravity.
Motion about the central longitudinal axis is called roll. A pitch movement is an up or down movement of the nose of the flight vehicle about a lateral axis through the CG. Pitch refers to the orientation about this lateral axis. A yaw motion is a side-to-side movement of the nose of the flight vehicle about a vertical axis through the CG. Yaw is also known as “heading.” These flight vehicle rotations are produced by torques or moments about these three axes commonly referred to as the body axes. The body axes may be represented by the letters x, y and z.
Typically thrust vectors of the engines of the flight vehicle are parallel to the centerline of the flight vehicle unless the CG is not along the centerline of the flight vehicle or if slight vehicle roll is desired. When each of the thrust vectors of the flight vehicle's multiple engines are parallel to the flight vehicle's centerline there is no moment about the CG. When engines of the flight vehicle are canted, then the thrust vectors of the canted engines are misaligned relative to the centerline of the flight vehicle. In such cases, however, the engines are canted to instead reduce moments about the CG. For example, some flight vehicles such as the space shuttle have misaligned mass properties relative to the centerline of the flight vehicle. The space shuttle had misaligned mass properties because of the external fuel tank and therefore the space shuttle's engines were canted to reduce the moments about the CG. Flight vehicles with misaligned mass properties may be referred to as asymmetrical flight vehicles. On the other hand, a symmetric flight vehicle has a CG that typically is located on the centerline of flight vehicle.
Some flight vehicles utilize thrust vector control (TVC) to control the trajectory and attitude of a flight vehicle by manipulating the direction of the thrust vector from one or more of the main engines relative to the CG. Thrust vectoring may be accomplished by gimballing the rocket engine. To gimbal, the nozzle of the rocket engine is rotated or swiveled about a pivot point from side-to-side to change the direction of the thrust vector relative the CG of the flight vehicle. Another method of TVC is to change the magnitude of a thrust vector of one engine relative to a thrust vector of one or more of the other engines to change the engine related moment about the CG of the flight vehicle. In both situations, the change in the thrust results in a moment about the centerline that changes the trajectory of the flight vehicle. However, TVC is limited to controlling the trajectory of the flight vehicle rather than controlling the attitude of the flight vehicle with moments. Also, the engines are not fixed relative the centerline of the flight vehicle during flight.
Where the thrust is oriented parallel to the centerline of the flight vehicle (roll axis), roll control is usually obtained by having at least one of the engines canted to impart roll upon the flight vehicle. In flight vehicles that fly outside the atmosphere, TVC is the primary means of control during main engine thrusting because aerodynamic control surfaces are ineffective. Also, on flight vehicles in space or low atmosphere environments when the main engines are not thrusting, moments are usually produced by a reaction control system (RCS) consisting of small rocket thrusters used to apply asymmetrical thrust on the flight vehicle.
However, it is desirable to obtain enhanced maneuverability about the pitch and yaw axes without gimballing of the main engines, or use of RCSs because of the additional costs associated with these systems and their operational use. It is also desirable to enhance pitch and yaw movements without dynamically canting the engines during flight.
It is with respect to these and other considerations that the disclosure herein is presented.